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Nitrogen limitation of microbial decomposition in a grassland under elevated
CO2

Subjects

Abstract

Carbon accumulation in the terrestrial biosphere could partially offset
the effects of anthropogenic CO2 emissions on atmospheric CO
2 (refs 1, 2).
The net impact of increased CO2 on the carbon balance of terrestrial
ecosystems is unclear, however, because elevated CO2 effects on
carbon input to soils and plant use of water and nutrients often have contrasting
effects on microbial processes3,4,5. Here we show suppression
of microbial decomposition in an annual grassland after continuous exposure
to increased CO2 for five growing seasons. The increased CO
2 enhanced plant nitrogen uptake, microbial biomass carbon, and available
carbon for microbes. But it reduced available soil nitrogen, exacerbated nitrogen
constraints on microbes, and reduced microbial respiration per unit biomass.
These results indicate that increased CO2 can alter the interaction
between plants and microbes in favour of plant utilization of nitrogen, thereby
slowing microbial decomposition and increasing ecosystem carbon accumulation.

Acknowledgements

We thank H. A. Mooney, C. Lund and B. A. Hungate for contributing to the
design and execution of the field experiment, H. L. Zhong for 15N
measurements, and P. Brooks for assistance with 13C measurements.
The Jasper Ridge CO2 experiment was supported by grants from the
National Science Foundation to the Carnegie Institution of Washington, the
University of California, Berkeley, and Stanford University. S.H. was supported
by the US NSF under a fellowship awarded in 1996.